Circadian (~24 hr) rhythms influence nearly all aspects of behavior and physiology. Disrupted circadian rhythms contribute to a wide range of human health conditions including sleep disorders such as shift work disorder and affective disorders such as anxiety and depression. However, understanding how circadian rhythms control or influence behavior remains a fundamental problem in neuroscience. While most neurons in the brain contain a molecular clock, overt circadian rhythms are orchestrated by the master pacemaker, the suprachiasmatic nucleus (SCN). Neurons of the SCN rhythmically fire action potentials and, together as a network, communicate circadian information to other circuits responsible for complex behaviors, perhaps the most obvious of which is sleep. Theoretical work has suggested that circadian drive for sleep (Process C) and homeostatic sleep pressure (Process S) follow distinct, albeit interconnected oscillations. A proposed mechanistic link between these processes lies in the hypocretin/orexin (Hcrt) system, a collection of neurons in the lateral hypothalamus that is essential for the stability of arousal. The loss of Hcrt or its receptors is associated with the disorder narcolepsy, which is characterized by the intrusion of sleep into wakefulness, and the optogenetic stimulation of Hcrt neurons directly promotes sleep-to-wake transitions. Importantly, Hcrt neuron activity (as read out by c-Fos), Hcrt transcript levels, and levels of Hcrt in the cerebrospinal fluid exhibit circadian rhythmicity. As circadian clock genes are ubiquitous throughout the brain and have been shown to influence neural activity and behavior outside the SCN, an intriguing possibility is that the molecular clock within Hcrt neurons regulates their activity independent of, or in concurrence with, input from the SCN. Thus, I hypothesize that SCN neural activity and the molecular clock within Hcrt neurons synergistically exert circadian control over Hcrt neural activity and sleep/wake behavior. To test this hypothesis, I will investigate the link between circadian rhythms, Hcrt neural activity, and sleep using a unique strategy that combines optogenetic manipulation of SCN activity, Hcrt neuron-specific molecular clock disruption, in vivo calcium imaging of Hcrt activity, and polysomnographic recording of sleep state. This proposal will provide novel insight into not only the essential relationship between circadian rhythms and arousal, but also the general neuroscience question of defining the neural mechanisms of complex behaviors that, when disrupted, can cause mental disease. To understand how the dysregulation of circadian rhythms may lead to illnesses such as sleep and affective disorders, it is first necessary to understand how they work in a healthy brain. Thus, this research plan will ultimately improve our understanding of the interplay of vital circadian and sleep circuits whose dysfunction can negatively impact human health.